1. Field of the Invention
This invention relates to an electrochemical apparatus capable of periodic operation without suffering long-term loss of efficiency or degradation of materials, in particular electrocatalyst layers or coatings.
2. Background of the Related Art
Ozone is known to be a powerful oxidizing species. Numerous methods and apparatus have been used to generate ozone and use ozone. However, many potential applications for the use of ozone do not require, or cannot utilize, a continuous stream of ozone gas or ozonated water. Unfortunately, the generation and use of ozone in discontinuous, unsteady-state, or batch modes of operation can be problematic for a variety of reasons. First, the high reactivity and rapid decomposition of ozone necessitate that the ozone be generated just prior to utilization. This dictates that ozone-generating capacity must closely match the peak rate of consumption. Second, the need to generate highly concentrated ozone favors electrochemical processes, most preferably using a lead dioxide anodic electrocatalyst.
While lead dioxide (PbO2) is generally unstable in aqueous acid solutions, the potential-pH equilibrium diagram for lead-water at 25° C. in FIG. 1 shows that maintaining a high positive electrode potential, with respect to the standard hydrogen electrode (SHE), on the positive PbO2 electrode in an electrochemical apparatus can stabilize lead dioxide. Therefore, lead dioxide instability in acid solutions is easily avoided in continuous electrochemical processes since a relatively high electrical potential is continuously maintained across the positive and negative electrodes. Where the electrochemical process is needed only periodically, it is possible to reduce the rate of the electrochemical process while supplying a lower electrical potential across the positive and negative electrodes, which stabilizes the lead dioxide by maintaining a trickle of electrical current between the electrodes. Unfortunately, continuous electrochemical processes and continuous electrical potentials are not practical in numerous applications, such as residential or consumer products, since power outages and even drained backup batteries can be experienced. Yet another possible solution is to use platinum metal as the anode electrocatalyst, but the penalty for using platinum is a lower ozone yield and higher cost.
Central to the operation of any electrochemical cell is the occurrence of oxidation and reduction reactions that produce or consume electrons. These reactions take place at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electrons transfer electric charge between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. Internal charge transfer is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short-circuiting of the cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode.
Electrochemical cells in which a chemical reaction is forced by added AC/DC electrical energy are called electrolytic cells. Electrochemical cells also include fuel cells, which are supplied with fuel to bring about the generation of DC current, and batteries, such as zinc/manganese dioxide.
The electrolyte may be a liquid electrolyte (aqueous or organic solvent, with a dissolved salt, acid or base) or a solid electrolyte, such as a polymer-based ion exchange membrane that can be either a cation exchange membrane (such as a proton exchange membrane, PEM) or an anion exchange membrane. The membrane may also be a ceramic based membrane, such as ytria-stabilized zirconia which is an O−2 ionic conductor.
However, ozone (O3) may be produced by an electrolytic process, wherein an electric current (normally D.C.) is impressed across electrodes immersed in an electrolyte. The electrolyte includes water that dissociates into its respective elemental species, O2 and H2. Under suitable conditions, the oxygen is also evolved as the O3 species. The evolution of oxygen and ozone at the anode may be represented as:2H2OO2+4H++4e−3H2OO3+6H++6e−
Utilization of high overpotentials, such as anode potentials greater than 1.57 Volts, and certain electrocatalyst materials enhance ozone formation at the expense of oxygen evolution. The water oxidation reactions yield protons and electrons which are recombined at the cathode. Electrons are conducted to the cathode via the external electronic circuit. The protons are carried through a solid electrolyte, such as a proton exchange membrane (PEM).
The cathodic reactions may utilize hydrogen formation:2H++2e−H2                or involve the reduction of oxygen as follows:O2+4H++4e−2H2OO2+2H++2e−H2O2        
Specialized gas diffusion electrodes are required for the oxygen reduction reaction to occur efficiently. The presence of oxygen at the cathode suppresses the hydrogen formation reaction. Furthermore, the oxygen reactions are thermodynamically favored over hydrogen formation. In this manner, the reduction of oxygen to either water or hydrogen peroxide reduces the overall cell voltage below that required to evolve hydrogen.
Therefore, there is a need for an electrochemical apparatus and methods that support periodic, non-steady state, or discontinuous operation without suffering degradation of materials, including electrocatalysts, or loss of efficiency. It would be desirable if the apparatus and methods did not require operator attention to verify the status of the power supply. It would also be desirable if the apparatus and methods support large amounts of repetitive use at various operating and standby durations and frequencies.